On Thu, Oct 29, 2020 at 7:15 AM Ananyev, Konstantin <konstantin.anan...@intel.com> wrote: > > > > > > > 29/10/2020 11:50, Andrew Rybchenko: > > > On 10/29/20 12:27 PM, Thomas Monjalon wrote: > > > > The mempool pointer in the mbuf struct is moved > > > > from the second to the first half. > > > > It should increase performance on most systems having 64-byte cache > > > > line, > > > > i.e. mbuf is split in two cache lines. > > > > On such system, the first half (also called first cache line) is hotter > > > > than the second one where the pool pointer was. > > > > > > > > Moving this field gives more space to dynfield1. > > > > > > > > This is how the mbuf layout looks like (pahole-style): > > > > > > > > word type name byte size > > > > 0 void * buf_addr; /* 0 + 8 */ > > > > 1 rte_iova_t buf_iova /* 8 + 8 */ > > > > /* --- RTE_MARKER64 rearm_data; */ > > > > 2 uint16_t data_off; /* 16 + 2 */ > > > > uint16_t refcnt; /* 18 + 2 */ > > > > uint16_t nb_segs; /* 20 + 2 */ > > > > uint16_t port; /* 22 + 2 */ > > > > 3 uint64_t ol_flags; /* 24 + 8 */ > > > > /* --- RTE_MARKER rx_descriptor_fields1; */ > > > > 4 uint32_t union packet_type; /* 32 + 4 */ > > > > uint32_t pkt_len; /* 36 + 4 */ > > > > 5 uint16_t data_len; /* 40 + 2 */ > > > > uint16_t vlan_tci; /* 42 + 2 */ > > > > 5.5 uint64_t union hash; /* 44 + 8 */ > > > > 6.5 uint16_t vlan_tci_outer; /* 52 + 2 */ > > > > uint16_t buf_len; /* 54 + 2 */ > > > > 7 struct rte_mempool * pool; /* 56 + 8 */ > > > > /* --- RTE_MARKER cacheline1; */ > > > > 8 struct rte_mbuf * next; /* 64 + 8 */ > > > > 9 uint64_t union tx_offload; /* 72 + 8 */ > > > > 10 uint16_t priv_size; /* 80 + 2 */ > > > > uint16_t timesync; /* 82 + 2 */ > > > > uint32_t seqn; /* 84 + 4 */ > > > > 11 struct rte_mbuf_ext_shared_info * shinfo; /* 88 + 8 */ > > > > 12 uint64_t dynfield1[4]; /* 96 + 32 */ > > > > 16 /* --- END 128 */ > > > > > > > > Signed-off-by: Thomas Monjalon <tho...@monjalon.net> > > > > > > I'd like to understand why pool is chosen instead of, for > > > example, next pointer. > > > > > > Pool is used on housekeeping when driver refills Rx ring or > > > free completed Tx mbufs. Free thresholds try to avoid it on > > > every Rx/Tx burst (if possible). > > > > > > Next is used for multi-segment Tx and scattered (and buffer > > > split) Rx. IMHO the key question here is we consider these > > > use cases as common and priority to optimize. If yes, I'd > > > vote to have next on the first cacheline. > > Between these two I also would probably lean towards *next* > (after all _free_ also has to access/update next). +1
> As another alternative to consider: tx_offload. > It is also used quite widely. > > > > > > > I'm not sure. Just trying to hear a bit more about it. > > . > > That's a good question. > > Clearly pool and next are good options. > > The best would be to have some benchmarks. > > If one use case shows no benefit, the decision is easier. > > > > If you prefer, we can leave this last patch for -rc3. > > >
